Laminated structure, and manufacturing method thereof

ABSTRACT

A laminated structure which has a substrate formed of a synthetic resin or glass and a silicon carbide coating layer formed by sputtering, wherein the light transmittance of the silicon carbide coating layer is 80% or less. Preferably, the silicon carbide coating layer has a light reflectance of 10 to 50%, the synthetic resin is polycarbonate, the impurity ratio on the surface of the silicon carbide coating layer is 1.0×10 12  atoms/cm 2  or less, and the silicon carbide layer has a thickness of 15 to 100 nm. The laminated structure is suitable for a recording medium such as a CD-ROM and a DVD-ROM since it has a silicon carbide layer excellent in oxidation resistance, chlorine resistance, moisture resistance, and the like.

TECHNICAL FIELD

The present invention relates to a laminated structure and amanufacturing method thereof. Particularly, the present inventionrelates to a laminated structure having a coating layer suitable for anoptical disk recording medium, and a method of manufacturing thelaminated structure.

BACKGROUND ART

As for read-only type optical disks of which CD-ROMs and DVD-ROMs arerepresentative, one method of increasing the recording capacity peroptical disk is a method of forming a recording layer, which includessignal information, as a two-layer structure. In a method in which alaser is irradiated from one side of an optical disk and what isrecorded in the disk is thereby read in the same direction, of twolayers, the layer closer to the side on which the laser is irradiatedneeds to be a semipermeable recording layer having some degree of lighttransmittance and light reflectance. Conventionally, this semipermeablerecording layer is obtained by using gold or silver as a target materialin a sputtering method and depositing it onto a substrate having convexand concave pits. The semipermeable recording layer formed of gold orsilver has a light transmittance and a light reflectance necessary forthe optical disk.

However, the semipermeable recording layer of this type has adisadvantage in terms of manufacturing cost since gold or silver isexpensive, and a problem of difficult handling arises because thespecific gravity of the semipermeable recording layer is large.Therefore, as a material for the semipermeable recording layer of theoptical disk, there has been a demand for a material to replace gold orsilver. Moreover, in the case of the semipermeable recording layer,since a thin layer of pure gold or silver is formed during sputtering,there is no way but to control the thickness of the semipermeablerecording layer in order to obtain light reflectance and the likenecessary for the semipermeable recording layer, and a problem arisesthat the semipermeable recording layer is not easily formed.

In general, sputtering is used to form the semipermeable recordinglayer. One of the reasons why sputtering is used is that, in order tolower the formation cost, physical methods such as sputtering, ionplating, vacuum deposition, and the like are advantageous over chemicalmethods of which a CVD method is representative, and among thesephysical methods, sputtering enables high-speed formation of thesemipermeable recording layer and excellent adhesion of thesemipermeable recording layer with respect to the substrate. In the caseof using the DC power system by which the semipermeable recording layercan be formed at high speed, the target material used for sputteringneeds to have such an electric conductivity that the volume resistivityis 10⁰ Ω·cm or less, and preferably about 10² Ω·cm or less.

An object of the present invention is to overcome the conventionalproblems described above, that is, to provide a laminated structurehaving a silicon carbide coating layer which has excellent oxidationresistance, chlorine resistance, moisture resistance, and the like, andis therefore suitable for an optical disk recording medium such as aCD-ROM, a DVD-ROM, and the like, in particular, and to provide a methodof manufacturing a laminated structure in which the aforementionedlaminated structure can be simply and reliably manufactured.

DISCLOSURE OF THE INVENTION

As a result of their thorough investigations for the purpose ofachieving the above object, the present inventors have learned thefollowing. As a result of seeking a substitute material for gold orsilver as a target material used in the sputtering method, they havefound that metals other than gold or silver are difficult to handlebecause many of them have a large specific gravity in general. Further,since a thin layer of a pure metal is formed by sputtering, the only wayto obtain the light reflectance and the like necessary for asemipermeable recording layer is to control the thickness of thesemipermeable recording layer, and manufacture thereof is thus not easy.On the other hand, many ceramic materials are generally insulatingmaterials, and their use as target materials is difficult. However, asintered silicon carbide used in the present invention has a volumeresistivity of 10⁰ Ω·cm or less, and therefore is effective as a targetmaterial which can control, by sputtering conditions, the opticalcharacteristics of the semipermeable recording layer formed in a thinshape.

Further, as the target material which can control the opticalcharacteristics of the semipermeable recording layer by sputteringconditions, a sintered silicon carbide of high density and high purityhaving a volume resistivity of 10⁰ Ω·cm or less is further effective.The sintered silicon carbide is obtained by carrying out, in a processfor producing a sintered silicon carbide disclosed in Japanese PatentApplication Laid-Open (JP-A) No. 10-67565 proposed by the presentinventors, a step of introducing a nitrogen-containing compound at thetime of preparing a mixture of a silicon carbide powder and anon-metallic sintering assisting material, or a step of introducing anitrogen-containing compound during preparation of a silicon carbidepowder at the time of mixing a carbon source and a silicon source, whichare the raw materials of the silicon carbide powder. Furthermore, asintered silicon carbide of high density and high purity, having animpurity ratio of less than 1.0×10¹¹ atoms/cm² on or near the surface,and having a volume resistivity of 10⁰ Ω·cm or less, is furthereffective. This impurity ratio on or near the surface is obtained bycleaning processes disclosed in Japanese Patent Application Nos.10-348569 and 10-348701 proposed by the present inventors.

Based on the above-described knowledge obtained by the presentinventors, the present invention includes the following aspects.

<1> A laminated structure which has a substrate formed of a syntheticresin or glass and a silicon carbide coating layer formed on thesubstrate by sputtering in which a sintered silicon carbide having avolume resistivity of 10⁰ Ω·cm or less is used as a target material,wherein the silicon carbide coating layer has a light transmittance of80% or less and a volume resistivity of 10⁰ Ω·cm or less.

<2> A laminated structure according to <1>, wherein the silicon carbidecoating layer has a light reflectance of 10 to 50%.

<3> A laminated structure according to <1> or <2>, wherein the syntheticresin is polycarbonate.

<4> A laminated structure according to any of <1> to <3>, wherein theimpurity ratio on the surface of the silicon carbide coating layer is1.0×10¹² atoms/cm² or less.

<5> A laminated structure according to any of <1> to <4>, wherein thethickness of the silicon carbide coating layer is 15 to 100 nm.

<6> A method of manufacturing a laminated structure having a substrateformed of a synthetic resin or glass and a silicon carbide coating layerformed on the substrate by sputtering in which a sintered siliconcarbide having a volume resistivity of 10⁰ Ω·cm or less is used as atarget material, wherein the silicon carbide coating layer is formed bycontrolling electric power inputted to a sputtering device, flow rate ofoxygen gas or nitrogen gas introduced, and sputtering time.

<7> A method of manufacturing a laminated structure according to

<6>, wherein the target material is a sintered silicon carbide, and thesintered silicon carbide has an impurity ratio of 1.0×10¹² atoms/cm² orless on and near the surface thereof, and a density of 2.9 g/cm³ ormore.

<8> A laminated structure according to any of <1> to <5>, wherein thedensity of the silicon carbide coating layer is 2.9 g/cm³ or more.

<9> A laminated structure according to any of <1> to <5> and <8>, usedas an optical disk recording medium.

PREFERRED EMBODIMENT FOR IMPLEMENTING THE INVENTION

Hereinafter, a laminated structure and a manufacturing method thereofaccording to the present invention will be described in detail.

The laminated structure according to the present invention has asubstrate and a silicon carbide coating layer on the substrate. Thelight transmittance of the silicon carbide coating layer is 80% or less.

The substrate is formed of a synthetic resin or glass.

Examples of the synthetic resin include polycarbonate, polymethylmethacrylate, polyolefine, and the like. Examples of the polyolefineinclude polyethylene, polypropylene, polybutene, and the like.

In the present invention, when used as a substrate material for DVDs orthe like, polycarbonate is particularly preferable among these materialsin terms of refractive index, light transmittance, thermal deformationtemperature, thermal conductivity, saturated water absorption, and thelike.

The silicon carbide coating layer is preferably formed by sputteringwith a sintered silicon carbide used as the target material.

The light transmittance of the silicon carbide coating layer is 80% orless, preferably 75% or less, and more preferably 70% or less.

The light transmittance is calculated as follows. For a complex in whicha silicon carbide coating layer having a thickness of 100 nm is formedon a glass substrate having a thickness of 1 mm, a light transmittancespectrum is measured using a spectrum photometer (manufactured byHitachi, Ltd., U-4000) while the wavelength of incident light is changedfrom 250 nm to 1000 nm. Subsequently, for a complex in which a 100 nmthick silicon carbide coating layer is formed on a 1 mm thick glasssubstrate which has been processed so that light does not reflect on therear surface, a light reflectance spectrum is measured using thespectrum photometer while the wavelength of incident light is changedfrom 250 nm to 1000 nm. From the light transmittance spectrum and thelight reflectance spectrum, a spectrum of a real number part ofrefractive indices and a spectrum of an imaginary number part of therefractive indices are calculated by a refractive index analyzer(manufactured by n & k Technology Co., Ltd., Iris 200). Then, bysubstituting the calculated spectrum of the real number part of therefractive indices and spectrum of the imaginary number part of therefractive indices at a wavelength of 633 nm of the incident light intothe following equation, the light transmittance (%) of the siliconcarbide coating layer of any thickness is obtained. The lighttransmittance described above refers to this light transmittance (%).The following equation takes into consideration an interference effectof repeated reflectance within a parallel plane layer.

Light transmittance${T\quad (\%)} = {\frac{e^{- {ad}}\left\{ {\left( {1 - r} \right)^{2} + {4r\quad \sin^{2}\varphi}} \right\}}{\left\{ {\left( {1 - {re}^{- {ad}}} \right)^{2} + {4{re}^{- {ad}}{\sin^{2}\left( {\varphi + \beta} \right)}}} \right\}} \times 100}$

α=4πκ/λ

β=2πn d/λ

r={(n−1)²+κ²}/{(n+1)²+κ²}

tan φ=2κ/(n²+κ²+1)

κ=k/n

In the above equation, λ represents the wavelength of the incident light(i.e., 633 nm), n represents the real number part of the refractiveindices, k represents the imaginary number part of the refractiveindices, and d represents the thickness of the silicon carbide coatinglayer.

When the light transmittance exceeds 80%, the amount of light reflectedfrom the semipermeable recording layer decreases, and thus it becomesdifficult to read information recorded on the semipermeable recordinglayer. This is not preferable since the rate of occurrence of errorsincreases.

A laminated structure having a silicon carbide coating layer with alight transmittance of 80% or less is preferable as a semipermeablerecording layer at which signal pits are formed in read-only typeoptical disks such as DVD-ROMs, DVD-VIDEOs, DVD-AUDIOs, and the like.

The light reflectance of the silicon carbide coating layer is preferably10 to 50%, more preferably 20 to 40%, and particularly preferably 30 to40%.

The light reflectance is calculated as follows. For a complex in which asilicon carbide coating layer having a thickness of 100 nm is formed ona glass substrate having a thickness of 1 mm, a light transmittancespectrum is measured using the spectrum photometer (manufactured byHitachi Ltd., U-4000) while the wavelength of incident light is changedfrom 250 nm to 1000 nm. Next, for a complex in which a 100 nm thicksilicon carbide coating layer is formed on a 1 mm thick glass substratewhich has been processed so that light does not reflect on the rearsurface, a light reflectance spectrum is measured using the spectrumphotometer while the wavelength of incident light is changed from 250 nmto 1000 nm. From the light transmittance spectrum and the lightreflectance spectrum, a spectrum of a real number part of refractiveindices and a spectrum of an imaginary number part of the refractiveindices are calculated by the refractive index analyzer (manufactured byn & k Technology Co., Ltd., Iris 200). Then, by substituting thecalculated spectrum of the real number part of the refractive indicesand spectrum of the imaginary number part of the refractive indices at awavelength of 633 nm of the incident light into the following equation,the light reflectance (%) of the silicon carbide coating layer of anythickness is obtained. The light reflectance described above refers tothis light reflectance (%). The following equation takes intoconsideration an interference effect of repeated reflectance within aparallel plane layer.

Light reflectance${R\quad (\%)} = {\frac{r\left\{ {\left( {1 - e^{- {ad}}} \right)^{2} + {4e^{- {ad}}\quad \sin^{2}\beta}} \right\}}{\left\{ {\left( {1 - {re}^{- {ad}}} \right)^{2} + {4{re}^{- {ad}}{\sin^{2}\left( {\varphi + \beta} \right)}}} \right\}} \times 100}$

α=4πκ/λ

β=2πn d/λ

r={(n−1)²+κ²}/{(n+1)²+κ²}

tan φ=2π/(n²+κ²+1)

κ=k/n

In the above equation, λ, n, k, and d are the same as described above.

If the light reflectance is less than 10%, the amount of light reflectedfrom the semipermeable recording layer decreases, and thus it becomesdifficult to read information recorded on the semipermeable recordinglayer, and the rate of occurrence of errors may increase. If the lightreflectance exceeds 50%, on the other hand, the amount of lightreflected from a total reflection recording layer, which is far from theside on which a laser is irradiated, decreases. As a result, it becomesdifficult to read information recorded on the total reflection recordinglayer, and the rate of occurrence of errors may increase.

The impurity ratio on the surface of the silicon carbide coating layeris preferably 1.0×10¹² atoms/cm² or less.

If the ratio exceeds 1.0×10¹² atoms/cm², defects may be easily caused onpits at the time of forming a disk, thereby increasing the rate ofoccurrence of errors.

The thickness of the silicon carbide coating layer is preferably 15 to100 nm.

If the thickness of the silicon carbide coating layer is less than 15nm, the light transmittance becomes large. If the thickness of thesilicon carbide coating layer exceeds 100 nm, on the other hand, timerequired for forming the silicon carbide coating layer becomessignificantly long. Therefore, neither case is preferable.

The laminated structure of the present invention can be preferablymanufactured by the method of manufacturing a laminated structureaccording to the present invention. This method will be describedhereinafter.

The method of manufacturing a laminated structure of the presentinvention is characterized in that a target material is used to form thesilicon carbide coating layer of the laminated structure of the presentinvention on the substrate by sputtering, and that electric powerinputted to a sputtering device, the flow rate of oxygen gas or nitrogengas introduced into the sputtering device, and sputtering time arecontrolled during the formation of the silicon carbide coating layer.

Sputtering methods depend on the conductivity of the target materialused. When the target material has low conductivity, high frequencysputtering, high frequency magnetron sputtering, or the like is used.When the target material has high conductivity, DC sputtering, DCmagnetron sputtering, or the like is used.

Among these sputtering methods, when a sintered silicon carbide is usedas the target material, DC sputtering or DC magnetron sputtering ispreferable since the target material is conductive.

In the case of DC sputtering which can form a coating layer at highspeed, the target material preferably has a volume resistivity of 10⁰Ω·cm or less and needs to have a conductivity of about 10⁻² Ω·cm orless.

As for the sintered silicon carbide used as the target material, asintered silicon carbide of high density and high purity having a volumeresistivity of 10⁰ Ω·cm or less is preferable. Such a sintered siliconcarbide can be obtained by carrying out, in the process for producing asintered silicon carbide disclosed in JP-A No. 10-67565 proposed by thepresent inventors, a step of introducing a nitrogen-containing compoundduring preparation of a mixture of a silicon carbide powder and anon-metallic sintering assisting material, or a step of introducing anitrogen-containing compound during preparation of a silicon carbidepowder at the time of mixing a carbon source and a silicon source, whichare the raw materials of the silicon carbide powder. More preferable isa sintered silicon carbide of high density and high purity, having animpurity ratio of less than 1.0×10¹¹ atoms/cm² on or near the surface,and having a volume resistivity of 10⁰ Ω·cm or less. This impurity ratioon or near the surface is obtained by cleaning processes disclosed inJapanese Patent Application Nos. 10-348570 and 10-348700 proposed by thepresent inventors.

A method of carrying out sputtering by using the sintered siliconcarbide as the target material will be described hereinafter.

Sputtering can be carried out in an atmosphere of inert gas such asargon and at an atmosphere pressure of 1.0×10⁻¹ to 1.0×10⁰ Pa afterintroducing inert gas.

Unlike gold or silver, the optical characteristics, such as the lighttransmittance, the light reflectance, and the like, of the siliconcarbide coating layer of the laminated structure of the presentinvention, which is manufactured by the method of manufacturing alaminated structure according to the present invention, can becontrolled by electric power inputted during sputtering, flow rate ofoxygen gas or nitrogen gas introduced (the introduction flow rate may bezero, that is, there may be no introduction), and sputtering time (thatis, the thickness of the silicon carbide coating layer to be formed).

Electric power inputted during sputtering depends on the area of thetarget material. The density of electric power inputted to the targetmaterial is represented by “electric power inputted/area of targetmaterial”. However, if the density of electric power inputted is toolarge, failure of the target material may be caused. Therefore, thedensity of electric power inputted is preferably 1.25 to 15.0 W/cm².

An example of the present invention will be described below. However,the present invention is not limited to this example.

Production of Target Material

The target material used herein is a sintered silicon carbide obtainedby the method described in Example 1 of Japanese Patent Application No.10-348700. The sintered silicon carbide has a density of 3.13 g/cm³, animpurity ratio of less than 1.0×10¹¹ atoms/cm² on or near the surface,and a volume resistivity of 3.2×10⁻² Ω·cm.

Sputtering Method

The target material having a thickness of φ100 mm×5 mm was loaded in asputtering device (SH-250, manufactured by ULVAC Japan, Ltd.). After theultimate vacuum within the sputtering device was adjusted to 7×10⁻⁵ Pa,argon gas was supplied into the sputtering device at a flow rate of 10cm³/minute. Sputtering time was adjusted so that the thickness ofrespective silicon carbide coating layers was set to values given in thefollowing Tables 1 and 2. Laminated structures, each of which has asilicon carbide coating layer formed a glass substrate having dimensionsof 5 cm×5 cm×1 cm and washed with a cleaning solution (produced by TamaChemicals Co., Ltd., TMSC), were produced.

The silicon carbide coating layers thus formed were respectivelymeasured by a coating thickness measuring profilometer (manufactured byRank Taylor-Hobson Co., Ltd., Talystep), and it was confirmed that theyeach had a desired thickness.

Evaluation Method

Oxidation resistance: The laminated structures were kept in an oxygenatmosphere at 50° C. for 1000 hours, and changes in the weights of therespective silicon carbide coating layers were measured.

Chlorine resistance: The laminated structures were kept in a chlorineatmosphere at 50° C. for 1000 hours, and changes in the weights of therespective silicon carbide coating layers were measured.

Moisture resistance: A silicon carbide coating layer was formed on aniron substrate, and this structure was kept in an atmosphere of ahumidity of 70% at 50° C. for 1000 hours. Changes in the iron substratewere observed by a light microscope with a magnification of ×1000.

The electric power inputted to the sputtering device, the flow rate ofoxygen gas and/or nitrogen gas introduced, and the sputtering time werechanged as shown in Tables 1 and 2, and silicon carbide coating layerswere respectively formed on glass substrates by sputtering. The ultimateatmosphere pressure before introduction of inert gas into the sputteringdevice was 7×10⁻⁵ Pa. The results are given in Tables 1 and 2.

TABLE 1 Electric Flow rate Flow rate Thickness of coating layers Weightchanges in Weight changes in power of O₂ of N₂ (nm) oxidation chlorineMoisture (W) (cm³/min) (cm³/min) 15 40 60 75 100 resistance test (%)resistance test (%) resistance test 1000 0.0 0.0 62.5 30.0 32.5 49.572.8 0 0 No change 500 0.0 0.0 62.5 30.0 32.4 49.1 71.9 0 0 No change100 0.0 0.0 63.2 30.5 32.5 48.4 73.0 0 0 No change 500 0.5 0.0 68.2 34.133.8 46.1 81.1 0 0 No change 500 1.0 0.0 77.8 43.5 38.7 44.8 76.0 0 0 Nochange 500 1.5 0.0 87.9 59.5 50.0 50.1 63.7 0 0 No change 500 2.0 0.093.9 75.1 64.6 61.4 64.8 0 0 No change 500 0.0 0.5 73.6 38.9 36.1 44.981.3 0 0 No change 500 0.0 1.0 79.3 45.3 39.8 45.1 74.5 0 0 No change500 0.0 1.5 83.2 50.7 43.4 46.5 69.2 0 0 No change 500 0.0 2.0 85.0 53.845.5 47.5 66.7 0 0 No change 1000 0.0 1.0 74.5 39.8 36.6 45.0 81.3 0 0No change 100 0.0 1.0 85.8 55.4 46.7 48.0 65.0 0 0 No change 1000 0.02.0 80.7 47.1 40.9 45.4 72.4 0 0 No change 100 0.0 2.0 88.7 61.4 51.551.1 62.6 0 0 No change Transmittance (%) (incident light having awavelength of 633 nm)

TABLE 2 Electric Flow rate Flow rate Thickness of coating layers Weightchanges in Weight changes in power of O₂ of N₂ (nm) oxidation chlorineMoisture (W) (cm³/min) (cm³/min) 15 40 60 75 100 resistance test (%)resistance test (%) resistance test 1000 0.0 0.0 33.6 66.5 63.1 41.9 7.00 0 No change 500 0.0 0.0 33.4 66.2 62.9 41.9 6.7 0 0 No change 100 0.00.0 32.7 65.6 62.7 42.6 5.2 0 0 No change 500 0.5 0.0 28.3 62.4 62.147.2 0.6 0 0 No change 500 1.0 0.0 19.6 53.4 58.0 50.9 13.5 0 0 Nochange 500 1.5 0.0 10.2 37.5 46.8 46.2 30.4 0 0 No change 500 2.0 0.04.8 22.4 32.5 35.4 31.0 0 0 No change 500 0.0 0.5 23.5 58.0 60.5 50.25.4 0 0 No change 500 0.0 1.0 18.3 51.8 57.2 50.9 16.3 0 0 No change 5000.0 1.5 14.8 46.5 53.7 50.0 23.7 0 0 No change 500 0.0 2.0 13.0 43.451.5 48.9 26.7 0 0 No change 1000 0.0 1.0 22.9 57.3 60.2 50.5 6.7 0 0 Nochange 100 0.0 1.0 12.0 41.4 49.9 48.0 27.9 0 0 No change 1000 0.0 2.017.0 50.0 56.0 50.7 19.1 0 0 No change 100 0.0 2.0 9.2 35.1 44.7 44.830.9 0 0 No change Reflectance (%) (incident light having a wavelengthof 633 nm)

Table 1 shows the light transmittance data, and Table 2 shows the lightreflectance data, of the silicon carbide coating layers in the laminatedstructures respectively manufactured, and the respective columns inTables 1 and 2 correspond to each other. In Table 1, the laminatedstructures whose light transmittance values given in thecolumn“Thickness of coating layers” exceed 80% correspond to ComparativeExamples, while the laminated structures whose light transmittancevalues given in the column are less than 80% correspond to Examples.

INDUSTRIAL APPLICABILITY

In accordance with the present invention, the conventional problemsdescribed above can be solved. Further, there can be provided alaminated structure, which has a silicon carbide coating layer excellentin oxidation resistance, chlorine resistance, moisture resistance, andthe like, and is therefore suitable for an optical disk recording mediumsuch as a CD-ROM, a DVD-ROM, and the like, and a method of manufacturinga laminated structure which enables simple and reliable manufacture ofthe laminated structure.

What is claimed is:
 1. A laminated structure comprising a substrateformed of a synthetic resin or glass and a silicon carbide coating layerformed on the substrate by sputtering, wherein a sintered siliconcarbide, the impurity ratio on or near the surface of the sinteredsilicon carbide being 1.0×10¹¹ atoms/cm² or less, and having a highdensity, a high purity and a volume resistivity of 10⁰ Ω·cm or less isused as a target material, wherein the silicon carbide coating layer hasa light transmittance of 80% or less and a volume resistivity of 10⁰Ω·cm or less and an impurity ratio of 1.0×10¹¹ atoms/cm² or less.
 2. Alaminated structure according to claim 1, wherein the silicon carbidecoating layer has a light reflectance of 10 to 50%.
 3. A laminatedstructure according to claim 1, wherein the synthetic resin ispolycarbonate.
 4. A laminated structure according to claim 1, whereinthe thickness of the silicon carbide coating layer is 15 to 100 nm.
 5. Alaminated structure according to claim 1, wherein the density of thesilicon carbide coating layer is 2.9 g/cm³ or more.
 6. A laminatedstructure according to claim 1, used as an optical disk recordingmedium.